Case 1: Victims at the park

This is where it gets a little grey area. Act under pressure is broad on purpose, so the same move that handles getting out of a burning building safe can handle picking a lock when a monster is searching for you (though if there was no monster normally you would just succeed because there is no pressure). This time, I'll say your act under pressure provides 1 more healing of harm to the teen, but normally the rules of healing state that you can only heal 1 harm with first aid, and then someone has to get hurt again for first aid to be used. Act under pressure never mentions being able to stabilize a wound, so only a hospital, Ajay's haven, or a move like the Divine's Lay On Hands or the Professional's Medic moves.

TLDR: I'll allow Ajay to heal 2 harm instead of 1 (margaret is performing first aid on Ajay instead of using magic to provide the other healing to the teen), but nothing performed stabilizes the teen. To satisfy the father, someone will need to help the teen to the hospital, or at the very least call an ambulance

On a different note, when any of you level up, 2 of the options you can take is to take a move from a different playbook. If you think it is important to be able to perform this kind of work in the future, or if Seth want to take it, you can use that option to take the Medic move from the professional. Keep in mind it doesn't say that you are acting under pressure, just that you roll +Cool, so your move to replace Cool in Act Under Pressure by RAW doesn't work, but we can talk about that

I healed Ajay. Seth and Ajay did some first aid on the boy. Can we call an ambulance for them? While we wait for the ambulance, can I use telepathy to make the boy and the father think that the boy was injured in some sort of accident? Like the boy, he tripped and cut his leg open on a sharp rock. Seth and Ajay saw it happen and were quickly healing the boy.

I think you can mess with them like that... Telepathy says you can put words in their head, not rewrite memory, so it won't be as perfect as that, but you can make them tell themselves it was a rock he tripped on instead of a knife that was thrown until they don't trust their memory very well.

Roll +Charm.
Success, they remember the knife and the rock, but tell themselves that they must be misremembering the event if they think someone would throw a knife in a forest.
Partial success, they remember the knife and the rock, but can't agree which is the truth but still remember if they try really hard
Failure, you reinforce the memory and they get angry when they see Margaret as they remember vividly she is who threw it

Small side note, I will say Seth and Ajay will see your eyes go black for just a little bit and know you did something, but what they do is up to them

Rolling Charm using Luck.

Ajay heals 1 harm but is still poisoned until he uses one of the two antivenoms he made earlier

We need to collect the other venom sack of the one we killed to make more antivenom. I say we kill this one, Regroup, make more antivenom, and then head to the last hunting spot.

As you made antivenom before, I'll say it is automatic to get the gland out and you know how to make 2 per gland. Since you rolled investigate a mystery, you can ask 2 questions from the move looking at the corpse. If it is something you can figure out with the coprse in front of you but no lab tools, I'll answer them.I may recommend "What sort of creature is it?" and maybe "What was it going to do?" but feel free to ask different questions.